Democritus first suggested the existence of the atom but it
took almost two millennia before the atom was placed on a solid
foothold as a fundamental chemical object by John Dalton (1766-1844).
Although two centuries old, Dalton's atomic theory remains valid
in modern chemical thought.

Dalton's Atomic Theory

1) All matter is made of atoms. Atoms are indivisible and
indestructible.

2) All atoms of a given element are identical in mass and
properties

3) Compounds are formed by a combination of two or more different
kinds of atoms.

4) A chemical reaction is a rearrangement of
atoms.

Modern atomic theory is, of course, a little more involved
than Dalton's theory but the essence of Dalton's theory remains
valid. Today we know that atoms can be destroyed via nuclear
reactions but not by chemical reactions. Also, there are different
kinds of atoms (differing by their masses) within an element
that are known as "isotopes", but isotopes of an element
have the same chemical properties.

Many heretofore unexplained chemical phenomena were quickly
explained by Dalton with his theory. Dalton's theory quickly
became the theoretical foundation in chemistry.